status: add `-out` flag to `status` command
status: add `-out` flag to `status` command

* Added `-out` flag to `dep status` command.
* The new flag accepts a single argument. Users can mention
  the file path where the status output should be written to.
* Blank values are ignored.
* When a file path is specified, the status output will be
  written to a file at the specified file path. If the file
  cannot be created (for some reason), a meaningful error
  message will be printed on the stdout.

Tests:
  - dep status -?
    Shows the information about the new -out flag
  - dep status -out junk/file/path
    A meaningful error is printed
  - dep status -out proper/file/path
    The status output is printed to the file at the
    mentioned file path.

golang/dep,golang/dep,golang/dep